Is there a rule of thumb for what should be the minimum exchange current to detect a certain redox couple in an aqueous solution?

I would like to make a rough calculations to see whether any of my hypothesis is feasible but I have no idea about the order of magnitude an exchange current should have in water. I could find a few examples for "normal" exchange currents for pH measurements or $\ce{Fe^{3+}/Fe^{2+}}$ but no minimum.

Background: I am trying to figure out, why one can measure a negative ORP in fermentations. There has been a lot of work done on the role of the microorganisms and their exofacial sulfhydryl groups. However, the question of WHAT is actually measured/happening at the electrode has not been answered.
I want to tackle this as part of my thesis and I can think of two mechanisms:
A) Direct interaction of sulfhydryl groups with the electrode surface
B) Mediation, e.g. via Fe or Mn ions.
My concerns about A are that cells are pretty big compared to ions so I am not sure whether enough cells could ever interact with the electrode to make this scenario realistic. In the case of B I am not sure if the concentrations are high enough.